In a fluidized medical bed containing granular constituents of very small dimensions (of the order of 80 to 150 microns, for example), waste from medical treatment and impurities accumulate and must be evacuated. Instead of emptying all of said partially soiled granular constituents and replacing them with a volume of fresh granular constituents, which is expensive, the invention proposes replacing only those constituents which contain waste or which are soiled.